A series of azido pyridine and adenine nucleotide photoaffinity analogues have been developed in this laboratory as general reagents for the investigation of the nucleotide binding regions of proteins. These compounds have been shown to act as competitive inhibitors of alcohol dehydrogenase and NADH-CoQ reductase (pyridine nucleotide analogues) and myosin and F1 (mitochondrial) ATPase (adenine nucleotide analogues) and following illumination to be irreversible inhibitors. The irreversible inhibition has been found to be associated with a covalent labeling of the proteins via the photogeneration of a reactive nitrene in the case of the myosin and F1 (ATPase) and presumably is responsible for the photodependent inhibition of NADH-CoQ reductase by arylazido pyridine nucleotides.